Chains and other linking devices, such as shackles, have portions or points which during use are subjected to significant amounts of wear. For example, linking members, including shackles and individual links of chains, operatively connected to a bucket of a dragline, such as that illustrated in FIGS. 1 and 2, have multiple wear points that as a consequence of use experience considerable wear.
Previously when one or more linking members of a chain exhibited considerable wear, the entire chain was removed so that the chain could be transported to a work shop, typically located a considerable distance from the dragline, where persons would grind off what remained of the existing wear crutches and welded new wear crutches into place. This is illustrated in FIG. 3.
The process for rebuilding wear crutches typically involved laying the chain on a bench, gouging out selected portions of each link where required, and preheating the links to a temperature of approximately 200° C. before welding individual pieces of metal rod to each link so as to replace the thickness of the portion of said link that had been worn away on one side of each link.
Instead of the links being arranged generally end to end, every second link was rotated approximately 90 degrees about an axis passing through an opening in said rotated link that was orthogonal to said link's longitudinal and transverse axes. This allowed persons to work on rebuilding wear crutches of adjacent links without additional movement of the links relative to one another.
Once the wear crutches had been rebuilt on one side of each link, the chain was rolled over so as to expose the opposite side of each link. The aforementioned process of rebuilding wear crutches was then repeated for the then exposed side of each link.
After all of the links had been rebuilt, they were typically gouged back to the original shape of a new crutch. The process to this point typically took approximately 9 to 10 hours to complete.
After the gouging process had been completed, a grinding process was typically employed to clean up the crutch area. The chains were then dye checked for cracks. If any cracks were found, the area surrounding the crack was gouged and rebuilt in a manner as previously described.
The time taken to check the rebuilt links and to rebuild those links that exhibited cracks often took an additional three or four hours to complete.
New chains typically last around eight weeks when used 24 hours a day, seven days a week before requiring their first rebuild. A hoist chain typically undergoes approximately eight rebuilds before it is scrapped. A rebuilt chain lasts around four weeks due to the weld material being softer than new chain material. Some mines wear the chains to destruction and don't rebuild the chains.
Instead, they replace destroyed chains with new chains every four to five months at a cost of approximately $14,000 per chain and two chains per machine.
There are numerous disadvantages with the existing method of rejuvenating or repairing chain links and other linking members that are provided with wear crutches. Most notably, the existing method is very time consuming and, as a consequence, rather expensive.
Further chain rebuilds need to be monitored constantly in case of cracking or the crutch wearing unevenly. Lastly, the capacity of a rebuilt chain to withstand wear is typically lower than that of a new chain due to the hardness of the weld material being around 240 bn which is much less than the hardness of the original chain, which is typically 300 bn.